An electronic camera can, for example, be used to digitally record image sequences which are later shown in a cinema. It is advantageous in this respect if the camera has high light sensitivity and simultaneously a high dynamic range. These two properties increase the quality of the recording and they help to reduce the costs for the illumination of the scene, for example.
In electronic cameras, the light sensitive elements or pixels convert light incident through an objective of the camera into electric signals. Each of the pixels is addressed to read out an image, with a pixel corresponding to a picture element of the image. The pixels arranged in rows and columns form an image field of the image sensor. A signal which is proportional to a charge of the pixel collected by an exposure is conducted to a data output of the image sensor.
Image sensors are in particular known which have a separate row selection line for each row and a separate column line for each column. The read-out of such an image sensor takes place row-wise, i.e. row for row. For this purpose, the pixels of the respective row are switched to the column lines by means of the respective row selection line. The column lines (also called bit lines) form the column bus. A separate column amplifier is usually associated with each of the column lines. A respective single column amplifier can, however, also be associated with a plurality of column lines. The column amplifiers are provided for the purpose of amplifying the signals of the pixels of the selected row applied to the column lines. The amplified signals are then clocked out, i.e. transferred from the column amplifiers to the data output or, if a plurality of data outputs are provided as is as a rule the case for reaching a higher frame rate, to the data outputs of the image sensor.
A separate data output is in this respect also as a rule not provided for each column line and/or for each column amplifier with a plurality of data outputs since a substantial space requirement and thus cost expense is associated with every data output. A multiplex device is therefore provided as a rule between column amplifiers and data outputs. For example, 32 data outputs (per channel) can be provided for an image sensor having 2880×2160 pixels. The number of data outputs is as a rule therefore smaller than the number of column lines.
To increase the dynamic range of the image sensor, provision can now be made that the signals of the pixels are read out by two or more channels which are separate from one another and which amplify the signals by different amounts, with each channel having a separate column amplifier and with the gain factors of the column amplifiers of the channels differing from one another. In such a case, the image sensor can include e.g. 2×32 data outputs, with the same column lines and/or column amplifiers being associated with the respective data output of the second channel as with the associated data output of the first channel.
The two channels can then be read out independently of one another and combined, with an image with a higher dynamic range arising overall. This is shown in FIGS. 1a and 1b. The combination of the amplified signals of two channels, of which one channel 101 has a high gain and one channel 103 has a low gain, takes place such that with a short exposure the amplified signal of the channel 101 with the high gain underlies an output value 105 and with a long exposure the amplified signal of the channel 103 with the low gain underlies an output value 105 for the respective picture element associated with the two channels 101, 103.
At the transition between short and long exposure, a simple switchover between the two channels 101, 103 is as a rule not sufficient since, due to the occurrence of manufacture-induced offset voltages which can usually not be avoided and/or deviations from the desired gains at the transition, a jump 107 in the exposure-output value characteristic would occur such as is shown in FIG. 1b. A cross-fading therefore usually takes place in a transition region 109 around the transition by which both the amplified signal of the channel 101 and the amplified signal of the channel 103 are taken into account, with the two signals being offset with respect to one another such that a gentle transition arises such as is shown in the enlarged representation of the transition in FIG. 1b. 
In the aforesaid example with 2880×2160 pixels and 2×32 data outputs, 90 read-out cycles are required to clock out the signals of the pixels of a row in two channels, provided that the read-out process includes precisely one read-out cycle for the respective pixel. The read-out of a row therefore requires 1.8 μs with a read-out clock frequency of 50 MHz. 3.888 ms are therefore required for the reading out of a whole image, i.e. of 2160 rows. A maximum frame rate of 257 Hz is thus possible.
An increase in the frame rate can generally be achieved in that the read-out clock frequency and/or the number of data outputs per channel is/are increased. An increase in the read-out clock frequency is, however, as a rule only possible if the respective next technology generation is used. An increase in the number of data outputs per channel is very cost-intensive.